1. Field of the Invention
The present invention relates to a method of manufacturing a crystalline semiconductor film containing silicon that is applied in an active layer of a thin film transistor (hereafter referred to as a TFT), and more particularly, to a spin addition method for a metallic element that has an effect of promoting crystallization. Further, the present invention relates to a semiconductor device having the crystalline semiconductor film.
2. Description of the Related Art
Recently, techniques of forming semiconductor integrated circuits by forming TFTs on an insulating substrate, such as a glass substrate, have been progressed rapidly, and electro-optical devices, typically active matrix liquid crystal display devices, utilizing these techniques have been put into practical use. In particular, active matrix liquid crystal display devices having integrated driver circuits are monolithic liquid crystal display devices in which a pixel matrix circuit and a driver circuit are formed on the same substrate, and the demand for these active matrix liquid crystal display devices has increased along with the demand for making them higher definition. In addition, developments are also advancing toward the realization of system on panels having built-in logic circuits such as xcex3 compensation circuits, memory circuits, and clock generator circuits or the like.
However, it is necessary that driver circuits and logic circuits operate at high speed, and therefore the application of amorphous silicon films to the active layers, which form regions such as channel forming regions, source regions and drain regions in TFTs, is unsuitable. TFTs having a polycrystalline silicon film as an active layer are coming into the mainstream at present. The application of low-cost glass substrates as substrates for forming TFTs is demanded, and the development of processes capable of being applied to glass substrates is flourishing.
For example, a technique is known in which a metallic element having a crystallization promotion effect, such as Ni (nickel) (hereafter referred to simply as a catalyst element) is introduced into an amorphous silicon film, and then a crystalline silicon film is formed by heat treatment. It is clear that crystallization is possible by heat treatment if a temperature on the order of 550 to 600xc2x0 C., less than the heat resistant temperature of the glass substrate, is used as the heat treatment temperature. It is necessary that the catalyst element be introduced into the amorphous silicon film with this crystallization technique. Methods such as plasma CVD, sputtering, evaporation, and spin addition can be given as introduction methods.
A spin addition method, in which a solution containing a catalyst element (hereafter referred to as a catalyst element solution) is added by spinning, is disclosed in JP 07-211636 A as a method of efficiently introducing a catalyst element into the vicinity of the surface of an amorphous silicon film. The spin addition method for the catalyst element solution as disclosed in the aforementioned unexamined patent application publication has the following characteristics:
(Characteristic 1) The amount of the catalyst element added to the surface of the amorphous silicon film can easily be controlled by controlling the concentration of the catalyst element within the catalyst element solution;
(Characteristic 2) The minimum amount of the catalyst element required in crystallization can therefore be added easily to the surface of the amorphous silicon film; and
(Characteristic 3) It is necessary to reduce the amount of the catalyst element within the crystallized crystalline silicon film as much as possible for reliability and electrical stability of the semiconductor device. The smallest amount of the catalyst element necessary for crystallization can be easily added by regulating the catalyst element concentration of the catalyst element solution with the spin addition method, and therefore the introduction of an excess amount of the catalyst element can be suppressed, which is advantageous for reliability and electrical stability of the semiconductor device.
The size of the glass substrates used in manufacturing of liquid crystal display devices has been becoming larger in view of the goal of applications to large size screens and increasing productivity. It has been projected that in the future, glass substrates that exceed 1 m on a side will be in use.
The above stated spin addition method for the catalyst element is one in which a liquid builds up on the substrate by dripping the catalyst element solution down onto the substrate surface, and the catalyst element solution that has been dripped down is then spun off by rotating the substrate at high velocity, thus adding a desired amount of the catalyst element to the substrate surface. This spin addition method is characterized in that the amount of the catalyst element added to the surface of the substrate can be easily controlled, and the like, and therefore it is a very important technique that is currently undergoing consideration for being put into practical use. However, there is a problem in that the uniformity of the amount of added catalyst element becomes poor as the substrate size becomes larger. In particular, the non-uniformity becomes a problem that cannot be ignored when the diagonal length of the square substrate is equal to or larger than 500 mm.
The main reason that the uniformity becomes poor is thought to be because at the spin drying state after the catalyst element solution has been applied to the substrate, the relative motion velocity with respect to air between the central portion of the substrate and regions in the periphery of the substrate differ. Caused by this, the evaporation speed of solvent components of the catalyst element solution varies within the surface of the substrate, and as a result, drying unevenness develop between the central portion and the peripheral regions.
FIG. 3 is a diagram showing the relationship between the size of the square substrate and the motion velocity at the edge portions of the substrate, and the following can be considered as causes of the generation of drying unevenness. For example, if the catalyst element solution is added to a 250 mm square substrate by spin addition, the motion velocity of the central portion of the substrate with respect to air is 0 m/min when the rotational velocity is 500 rpm (500 rotations/minute), while the edge portions of the substrate rotate at a motion velocity of approximately 400 m/min. Motion with respect to air thus becomes higher speed with increasing distance from the central portion of the substrate, and therefore friction with the air becomes severe, and the solvent components of the catalyst element solution evaporated very rapidly. Drying unevenness therefore develop due to the differences in evaporation speed of the solvent components between the central portion of the substrate and the edge portions of the substrate.
In addition, the drying unevenness caused by the different drying speeds of the solvent components tend to manifest at corner regions of the square substrate. It is thought that this is because air is pushed aside along with rotational motion in the corner regions of the substrate, and therefore the friction with the air becomes exceptionally severe there. These types of drying unevenness are large problems that influence the amount of deposited catalyst element, and that influence various fluctuations, such as fluctuations in the final crystallization ratio, the size of crystal grains, and their alignment after crystallization.
The present invention has been made in view of solving the above problems. Specifically, an object of the present invention is to resolve problems with uniformity in the amount of added catalyst element within a substrate, caused by drying unevenness in a spin addition method.
As stated above, there is a fear of a problem of non-uniformity in the amount of added catalyst element within a substrate, caused by drying unevenness during spin drying of a catalyst element solution in a spin addition method for a metallic element (catalyst element) for promoting crystallinity. In order to resolve the non-uniformity in the amount of added catalyst element within the substrate, it is necessary to eliminate the drying unevenness that occur during spin drying and which are surmised to be the cause of the non-uniformity. The drying unevenness during spin drying are thought to occur due to the development of a difference in evaporation speed for solvent components that accompanies friction with the air when the substrate is rotating.
In order to solve the above-mentioned problems and, in the spin addition process for the catalyst element, to improve the uniformity within the substrate of the amount of the catalyst element deposited thereon. The present invention takes a measure in which the rotational acceleration speed up through a switch over to high velocity rotation is optimized in accordance with the substrate size, thereby improving the uniformity of the amount of added catalyst element within the substrate.
Specifically, a method of manufacturing a crystalline semiconductor film has: a first step of depositing an amorphous semiconductor film containing silicon on an insulating substrate; a second step of adding a catalyst element for promoting crystallization to the entire surface of the amorphous semiconductor film by a spin addition method; and a third step of forming a crystalline semiconductor film containing silicon by heat treating the amorphous semiconductor film; in which the spin addition method for the catalyst element is performed with a rotational acceleration speed from 5 to 120 rpm/sec. Alternatively, the rotational acceleration speed y in the spin addition process for the catalyst element is determined by the equation y less than =Axxe2x88x92B (where x is the diagonal size of the substrate and A and B are constants).
Note that, in the case of adding the crystallization promoting catalyst element by the spin addition method, an addition method in accordance the following spin addition method may also be employed. A mask insulating film may be deposited onto an amorphous semiconductor film, and an opening region may be formed in a portion of the mask insulating film, after which the crystallization promoting catalyst element may be added to the mask insulating film by a spin addition method. The spin addition method for the catalyst element is performed at a precondition of a maximum fixed rotational velocity value of 800 to 1200 rpm. Addition of the solution containing the catalyst element is performed by dripping the solution during acceleration or during constant velocity rotation of the substrate, distributing the catalyst element over the entire surface.
Compared to a circular shape substrate, uniformity becomes poorer in the case where the diagonal length of the square substrate is equal to or greater than 500 mm with a conventional spin addition method. However, uniformity can be improved even if the diagonal of the substrate is equal to or greater than 500 mm by applying the aforementioned structure of the present invention. The amount of fluctuation in the amount of added catalyst element within the substrate is lowered in the case where the catalyst element is added by the spin addition method, and therefore the uniformity in the crystallization ratio after crystallization, the grain size, the grain arrangement, and the like can be enhanced, and a uniform crystalline semiconductor film can be formed over the entire surface of a large surface area substrate.